1. Field of the Invention
The subject invention is directed generally to a system for delivering fluid flow, and more particularly, to a system for using two pumps to efficiently and quickly respond to high transient flow demands at a high pressure without excessive size or weight.
2. Background of the Related Art
In modern gas turbines, fuel pumps need to supply fuel to engine geometries. For example, in mid to larger class engines, linear pistons are used as guide vanes. The linear pistons require a significant source of fuel to slew. This slewing is a transient event that can unacceptably starve the supply of fuel to the engine. Thus, fixed delivery fuel pumps have often been sized to provide excessive fuel flow capacity in order to insure adequate supply to the associated engine. Under most operating conditions, this results in large amounts of pressurized fuel being returned to the pump inlet for recirculation. The return and recirculation results in significant fuel heating due to additional energy being put into the fuel which is subsequently turned into heat as the pressure drops in the recirculation path. In modern designs, fuel heating is a critical issue because the fuel is typically used as a heat exchanger to maintain proper operating temperature. Other methods of heat exchange are undesirable because of the associated size, weight and cost.
Variable displacement fuel pumps have partially overcome the thermal deficiencies of fixed delivery pumps by being able to vary the amount of fuel output. By varying the fuel output, the fuel delivered more closely matches engine demand. Thus, the recirculated flow, along with the heat generated thereby, is reduced. Variable displacement fuel pumps are known in the art as disclosed in U.S. Pat. No. 5,833,438 to Sunberg and U.S. patent application Ser. No. 10/705,362 filed on Nov. 11, 2003, each of which is herein incorporated by reference. A variable displacement pump typically includes a rotor having a fixed axis and pivoting cam ring. Controlling the position of the cam ring with respect to the rotor controls the output of the pump. As a result, the output flow may be controlled by a torque motor operated servo valve acting on the cam ring.
However, the engine operating conditions include transients such as those caused by engine actuator slewing, start-up and the like as would be appreciated by those of ordinary skill in the pertinent art. Under such rapidly varying operating conditions, prior art pump control systems often utilize a portion of the fuel supply to move the cam ring. Because a portion of the supply is subtracted to reset the pump, the resulting response is slow and creates flow disturbances to the engine. Moreover, many prior art pump control systems lack the required stability to reliably provide fuel to the engine. So despite the advances of the state of the art, variable displacement pumps are lacking in stability and still do not respond quickly enough to varying engine demands. As a result, poor performance and inaccurate fuel flow are still common.
Examples of variable displacement pump control arrangements are disclosed in U.S. Pat. Nos. 5,716,201 to Peck et al. and 5,715,674 to Reuter et al., the disclosures of which are herein incorporated by reference in their entirety. These pump control systems attempt to maintain accurate fuel flow throughout the range of engine operating conditions. However, as noted above, such systems still contain inadequacies such as complexity, thermal inefficiency and abrupt shutdowns. It is also undesirable for pump control systems to include sophisticated electronics and numerous additional components that undesirably increase costs and complexity. Still further, these arrangements are often undesirably large. These disadvantages are further magnified in high pressure fueldraulic applications where the fuel pump has to be sized to provide engine geometry and variable nozzle actuators at relatively high pressure (such as 3000-5000 psid) and at the same time provide very fast dynamic response. In such situations, building high capacity pumps has proved difficult and poor performance is typical.
In view of the above, it would be desirable to provide a flow control system which has a robust design for fast response, small size and weight, and accurate regulating of the output with stability and without the associated drawbacks of the prior art.